IIHS research bibliography

IIHS has been conducting research for more than 50 years. Papers published in copyrighted publications such as books, journals and conference proceedings are available upon request, but their contents may not be redistributed or republished without consent of the publishers. Unpublished and noncopyrighted reports are available for download, and their contents may be redistributed and republished with attribution.

Detecting radar detectors: on and off the road with the VG-2

Lund, Adrian K.Insurance Institute for Highway Safety February 1993

Abstract

No matter what their owners say, the only plausible reason for using a radar detector is to break the speed limit with impunity. Drivers who can answer anonymously will often admit this. In a national telephone survey, more than half the drivers who used a radar detector said they drove faster than they would without it (and 75 percent said they believed it had kept them from getting a speeding ticket in the previous year) (IIHS, 1998). Truck drivers, particularly, have been show to drive faster with radar detectors. On interstates with 65 mph speed limits, more than twice as many trucks with radar detectors as without were observed traveling at 70 mph or more; three times as many were seen to be doing 75 mph or more (Teed and Williams, 1990; Williams et al., 1990). Of course, the use of radar detectors does not mean absolute freedom from speeding tickets. The siting of enforcement radar over hills or around curves and the growing use of instant-on radar, laser devices, and VASCAR limit the usefulness of radar detectors. Nevertheless, the widespread use of radar speed enforcement means that drivers with radar detectors are ofen able to slow down before officers can obtain an objective measure of their speed, and these drivers frequently avoid tickets. In most states, and in most vehicles, radar detectors are perfectly legal. They are not permitted in any vehicle in the District of Columbia or Virginia, however, and two states, Illinois and New York, ban their use in commercial vehicles or vehicles over a certain size. More to the point, the Federal Highway Administration is contemplating a nationwide ban on radar detectors in heavy trucks. Until recently, efforts to enforce such bans had to rely on officers' visual checks of a vehicle as it went by (not very efficient) or after it had been stopped for some other reason (not very efficient either). A few years ago, however, a Canadian company developed a radar detector detector (RDD) - an electronic device that can isolate and identify the characteristics microwave radiation emitted by radar detectors. Called the Interceptor VG-2, the device responds to a nearby radar detector with an audible beep that sounds more rapidly (like a geiger counter) and a display that varies with the intensity of the microwave energy detected by the device. It is at this time the only device of its kind on the market.

The duration of speed reductions attributable to radar detectors

The duration of speed reductions attributable to radar detector use was observed. When the entire traffic stream was exposed to police radar, the proportion of vehicles traveling more than 10 mph over the speed limit dropped initially from 42% to 28%, but by one mile after exposure, this percentage had already increased to 38%. When speeding vehicles with radar detectors (about 45% of all speeding vehicles) were exposed to police radar, speeds dropped by approximately 15%, but by one mile after exposure, nearly half of the reduction was recovered. These findings indicate that radar detector users slow only briefly when alerted to police radar and that radar detectors are use primarily to avoid speed limit enforcement.

Radar detector use in trucks in 17 states

Radar detector use and speeds of large trucks were measured on interstate highways in 17 states throughout the continental United States. Data were collected from a passenger van traveling below the truck speed limit using a "radar detector detector" (RDD) device to determine radar detector use and nondetectable radar to measure speeds. At least 52% of all trucks and 50% of trucks carrying hazardous materials were using radar detectors. Among tractor-semitrailers, at least 59% were using radar detectors with rates ranging from 39% in California to 69% in Oklahoma. Trucks with radar detectros were more likely than those without radar detectors to be traveling at illegal speeds. Overall radar detector use among trucks in these largely midwestern and western states was considerably higher--about one and a half times--than that observed in an earlier study conducted in seven eastern states.

Radar detector use in large trucks

Radar detector use and speeds of large trucks were measured on interstate highways in seven eastern states. Radar detector use is legal in five of the states and illegal in Connecticut and Virginia. Data were collected from a passenger van traveling slower than the truck speed limit. A "radar detector detector" (RDD) device was used to determine radar detector use, and nondetectable radar was used to measure speeds of trucks that passed the van. Overall, at least 36% of trucks observed were using radar detectors and at least 40% of tractor-semitrailers were using radar detectors. At least 46% of the trucks carrying hazardous materials were using radar detectors. Radar detector use rates ranged from at least 33% in Maryland and Virginia to 47% in Delaware. Trucks with radar detectors in use (including hazardous materials carriers) were more likely than those without to be traveling at excessive speeds. Previous surveys in Maryland and Virginia yielded no evidence that Virginia's radar detector ban was effective. The present survey results, which included more highways in Virginia, suggest that the radar detector bans in Virginia and Connecticut may reduce use slightly.

Radar detector use and speeds in Maryland and Virginia

Radar detector use and vehicle speeds were determined at four rural interstate sites in Maryland and Virginia. Radar detectors are legal in Maryland but banned in Virginia. Receivers designed to pick up the microwave signals that emanate from radar detectors were used to indicate the presence of radar detectors in use; speeds of free-flowing vehicles were measured using nondetectable radar. In both Maryland and Virginia, tractor-semitrailer trucks were most likely to have radar detectors in use (28% in Maryland, 32% in Virginia). Four percent of passenger vehicles in Maryland and five percent in Virginia had radar detectors in use. The estimates of radar detectors in use in this survey are conservative because under certain traffic conditions it was not possible to determine whether only one or more than one vehicle had a radar detector in use. For all categories of vehicles, those with radar detectors in use were much more likely than those without to be traveling at excessive speeds; the higher the speed the more likely it was that vehicles had radar detectors in use.

A survey about radar detectors and driving behavior

Opinion Research CorporationInsurance Institute for Highway Safety October 1988

Abstract

This report presents the findings of a telephone survey on the use of radar detectors. The survey was conducted among a national probability sample of 503 men and 505 women (Wave I - August 1988) and 500 men and 507 women (Wave II - September 1988) living in private households in the continental United States.

A critique of "A comparison of the automobile accident rates of radar detection device users and non-users"

Lund, Adrian K.Insurance Institute for Highway Safety May 1988

Abstract

A report based on a sample of 870 radar detector users and 930 drivers in the general public was prepared by Yankelovich, Skelly and White/Clancy Shulman, Inc., for RADAR, Inc., which concluded "radar detection users are at least as safe drivers as nonusers, as measured by the miles driven per accident experienced." A critical examination of this report reveals serious sampling and analytic flaws. The radar detector user sample is unlikely to represent all users due to the manner in which it was obtained, and the general public sample is an inappropriate comparison group with driving patterns and mileage differing from the user group. As presented, the survey cannot answer the question of whether radar detectors increase the crash likelihood of their users.

Radar detectors and speeds in Maryland and Virginia

This study measured the extent to which vehicles violating speed limits in Maryland (where radar detectors are legal) and Virginia (where radar detectors are illegal) slow down when a police radar unit is activated and the extent to which overall vehicle speeds are affected due to some vehicles having radar detectors. Data collected in Maryland covered speed distributions in the presence and absence of conventional police radar as measured by an automatic speed monitoring station, speed distributions in the presence or absence of police radar as measured by nondetectable radar, and speed changes among fast vehicles as measured by nondetectable radar when police radar was suddenly activated. Data collection procedures were similar in Virginia except that speed distributions from monitoring stations were not collected. Findings indicate that overall travel speeds are lower and the proportion of vehicles traveling at very high speeds are lower when hidden police radar units are operating. Tractor trailers showed the greatest declines in average speed and in the proportion exceeding 65 mph in the presence of continuously operating detectable police radar. Results suggest that radar detectors are being widely used to elude police speed enforcement and that travel speeds on U.S. highways are faster than they would be in the absence of radar detectors.

Prohibiting radar detectors: legal issues

No single method will minimize the deaths and Injuries that result from motor vehicle crashes. Countermeasures must be applied that not only reduce the incidence of crashes but also reduce the damage that results. Excessive speed is undeniably a major contributing factor to crash losses both as a cause of many crashes and by increasing the severity of crashes. Speed control is therefore a vital part of efforts to assure the safety of all highway users. Consistent and intensive enforcement of speed laws is a key to controlling excessive speed.l The proliferation of radar detectors is inhibiting this effort. Radar detectors undermine police ability to identify vehicles that are speeding and to measure the speeds at which these vehicles are moving.2 Connecticut, Virginia, and the District of Columbia prohibit the use of radar detectors (by statute in Virginia and by regulation in Connecticut and the District of Columbia). This paper reviews these prohibitions and courts' interpretations of them; explores the constitutional, statutory, and regulatory law that is relevant to radar detector bans; and proposes a theoretical justification for the banning of radar detectors.

Automated traffic enforcement: responding to the critics

In many countries, traffic volumes and the number of drivers are rising faster than the availability of police officers whose routine duties include traffic law enforcement. Automated traffic enforcement, which produces photographic evidence of vehicles detected speeding or running red lights, can be used to supplement traditional enforcement. In the United States and Canada, a number of individuals and organizations have been very vocal in their opposition to automated traffic enforcement. They argue that automated enforcement programs are unnecessary for improving road safety, that they unfairly target relatively good drivers, and that they are motivated by revenue generation rather than safety. These arguments, however, often ignore the numerous peer-reviewed studies that have found real-world benefits in communities that use automated enforcement — cameras deter would-be violators, reduce crashes, and save lives. Solid, published research by a number of experts demonstrates that red light cameras save lives, and speed cameras substantially reduce speeding and speed-related crashes. Surveys of drivers and other road users indicate widespread support for automated enforcement. With regard to fairness, the objective of photo enforcement is to deter violations, not to surreptitiously catch violators. The more public the enforcement is, the better. If anything, automated enforcement programs improve fairness by reducing the potential for prejudicial enforcement. Finally, photo enforcement is intended to improve traffic safety by modifying the driver behaviors that lead to crashes, and it is reasonable to expect that people who break the law should pay for enforcing it. Ticket revenue should decline over time as the cameras succeed in deterring would-be speeders and red light runners. This paper provides research-based responses to the critics’ arguments as well as best practice guidelines for effective automated enforcement programs.

A point-by-point response to “Speed or Greed: Does Automated Traffic Enforcement Improve Safety or Generate Revenue?”

Farmer, Charles M.Insurance Institute for Highway Safety August 2016

Abstract

In December of 2015, the Frontier Centre for Public Policy released a policy paper authored by Hiroko Shimizu and Pierre Desrochers entitled “Speed or Greed - Does Automated Traffic Enforcement Improve Safety or Generate Revenue?” The report concludes that automated enforcement programs often are unnecessary for improving road safety. The report, however, is biased toward apparently preconceived positions on the acceptability of automated enforcement. It ignores numerous peer-reviewed studies that have found real-world benefits in the communities that use automated enforcement — cameras deter would-be violators, reduce crashes, and save lives. The authors provide relatively few citations of research from peer-reviewed journals, and some of the research they cite was either misinterpreted or misrepresented. As such, this so-called “concise summary of the key facts” can only be viewed as incomplete. Solid, published research by a number of experts demonstrates that red light cameras save lives, and speed cameras substantially reduce speeding and speed-related crashes. National surveys indicate widespread support for automated enforcement. The following lists a number of quotes from the paper (in italics) along with opposing views, research, and details that the authors failed to include. Section headings are as given by Shimizu and Desrochers.

Effects of automated speed enforcement in Montgomery County, Maryland, on vehicle speeds, public opinion, and crashes

Hu, Wen; McCartt, Anne T.Traffic Injury Prevention (TIP) 2016

Abstract

Objectives: In May 2007, Montgomery County, Maryland, implemented an automated speed enforcement program, with cameras allowed on residential streets with speed limits of 35 mph or lower and in school zones. In 2009, the state speed camera law increased the enforcement threshold from 11 to 12 mph over the speed limit and restricted school zone enforcement hours. In 2012, the county began using a corridor approach, in which cameras were periodically moved along the length of a roadway segment. The long-term effects of the speed camera program on travel speeds, public attitudes, and crashes were evaluated.

Methods: Changes in travel speeds at camera sites from 6 months before the program began to 7½ years after were compared with changes in speeds at control sites in the nearby Virginia counties of Fairfax and Arlington. A telephone survey of Montgomery County drivers was conducted in Fall 2014 to examine attitudes and experiences related to automated speed enforcement. Using data on crashes during 2004–2013, logistic regression models examined the program's effects on the likelihood that a crash involved an incapacitating or fatal injury on camera-eligible roads and on potential spillover roads in Montgomery County, using crashes in Fairfax County on similar roads as controls.

Results: About 7½ years after the program began, speed cameras were associated with a 10% reduction in mean speeds and a 62% reduction in the likelihood that a vehicle was traveling more than 10 mph above the speed limit at camera sites. When interviewed in Fall 2014, 95% of drivers were aware of the camera program, 62% favored it, and most had received a camera ticket or knew someone else who had. The overall effect of the camera program in its modified form, including both the law change and the corridor approach, was a 39% reduction in the likelihood that a crash resulted in an incapacitating or fatal injury. Speed cameras alone were associated with a 19% reduction in the likelihood that a crash resulted in an incapacitating or fatal injury, the law change was associated with a nonsignificant 8% increase, and the corridor approach provided an additional 30% reduction over and above the cameras.

Conclusions: This study adds to the evidence that speed cameras can reduce speeding, which can lead to reductions in speeding-related crashes and crashes involving serious injuries or fatalities.

Objective: Pedestrians represent more than one third of all traffic deaths in Washington, D.C. The District plans to expand its long-standing automated traffic enforcement program in 2013 from speed and red light cameras to cameras to enforce pedestrian right-of-way laws at crosswalks and stop sign laws. This study collected information on the opinions, behaviors, and knowledge of D.C. residents related to camera enforcement and pedestrian safety issues.

Methods: A telephone survey of 801 adult D.C. residents was conducted in November 2012 with approximately equal numbers of respondents in each of D.C.’s eight wards. Quotas were used to ensure that the sample was representative of the demographic characteristics of adults in each ward. For analyses combining responses across the wards, data were weighted to correspond with the demographic characteristics of adults in the city.

Results: Most respondents believed that drivers speeding, running red lights, running stop signs, and not stopping for pedestrians are serious threats to their safety. Respondents strongly supported the speed and red light camera programs, with 76 percent of respondents favoring speed cameras and 87 percent favoring red light cameras. Support was more limited for the camera enforcement that was not yet in place at the time of the survey, with 50 percent of respondents favoring stop sign cameras and 47 percent of respondents favoring crosswalk cameras. Twenty-four percent of respondents had not driven a car in D.C. in the past month, and higher proportions of these nondrivers favored speed cameras (90%), stop sign cameras (67%), and crosswalk cameras (59%) than respondents who drove in D.C. in the past month. Respondents who supported camera enforcement cited safety as their main reason. More than 9 in 10 respondents knew that D.C. law requires drivers to stop for pedestrians crossing the street in marked crosswalks at intersections without traffic signals and midblock, but only 54 percent knew that drivers must stop for pedestrians crossing the street at intersections without marked crosswalks.

Conclusions: Most residents supported speed cameras and red light cameras, but support was lower for stop sign and crosswalk cameras. Emphasizing the safety benefits of stop sign and crosswalk cameras and documenting the extent of safety problems at stop signs and crosswalks may increase support for these new forms of camera enforcement. Communities considering automated enforcement should take into account the opinions of pedestrians, even though they are not subject to camera citations.

Evaluation of automated speed enforcement in Montgomery County, Maryland

Objectives: Almost one quarter of speeding-related fatalities occur on streets with speed limits of 35 mph or less. In 2007, Montgomery County, Maryland, implemented the state's first automated speed enforcement program, with camera use limited to residential streets with speeds limits of 35 mph or less and school zones. The purpose of the present study was to evaluate initial effects of camera enforcement on traffic speeds and to assess public attitudes.

Methods: Vehicle speeds were measured approximately 6 months before and 6 months after speed cameras were deployed and warning signs were installed. Speed data were collected on a sample of roads with and without camera enforcement in Montgomery County, as well as on a sample of roads in a comparison community that did not have speed cameras. In addition, telephone surveys were conducted in Montgomery County.

Results: Relative to speeds of drivers on roads in the comparison community, the proportion of drivers in Montgomery County traveling more than 10 mph above posted speed limits declined by about 70% at locations with both warning signs and speed camera enforcement, 39% at locations with warning signs but no speed cameras, and 16% on residential streets with neither warning signs nor speed cameras. Public opinion surveys found 74% of Montgomery County drivers thought speeding on residential streets was a problem. Six months after enforcement began, 60% of drivers were aware of the camera program and 62% supported it.

Conclusions: The camera program was effective at reducing speeding on targeted streets. The finding of speed reductions beyond targeted locations is evidence that highly visible automated enforcement can promote community-wide changes in driver behavior. Although a majority of drivers supported automated speed enforcement, about one third opposed it. Jurisdictions planning to implement speed cameras should draw on international experience to anticipate controversies that generally arise and take steps to address them.

Speed cameras can reduce speeding and injury crashes, but in many communities they are confined to low-speed settings such as residential streets and school zones. In 2006 the city of Scottsdale, Arizona, implemented a 9-month pilot program to evaluate the feasibility and effects of highly visible speed camera enforcement on a busy urban freeway. This was the first use of fixed speed cameras on a major US highway. Deployment of six cameras along an 8-mile corridor was associated with large declines in mean speeds and an 88% decrease in the odds of vehicles traveling 11mph or more above the 65mph limit. Traffic speeds increased soon after the pilot program was suspended. In addition to reducing speeding along the enforcement corridor, speed cameras were associated with large reductions in speeding on the same highway but 25 miles away from the camera installations. However, traffic speeds were fairly stable on urban freeways in Scottsdale that were not part of the study road. Public opinion surveys found widespread concerns about speeding on the Loop 101 freeway and high levels of support for speed camera enforcement on this road.

Stop for the camera: Study shows automated enforcement does work, and it is legal

Fields, MicheleTransportation Management & Engineering July 2008

Abstract

Vehicle travel has increased dramatically in recent years, but traffic law enforcement resources have not kept pace. Between 1995 and 2005, the estimated number of vehicle-miles traveled In the U.S. increased by 23%, but the number of municipal law enforcement officers grew by 12%. The use of camera technology to enforce traffic laws frees police to handle other pressing Issues.

Controversies and speed cameras: lessons learnt internationally

Research has shown that speed cameras reduce motor vehicle speeds and crashes. Cameras havebeen used extensively in jurisdictions such as Victoria, Australia, and Great Britain, and much less oftenin the United States and Canada. They have been controversial wherever used. We describe thedevelopment of camera programs in Victoria and Britain and discuss the types of controversies that havearisen, the techniques used to address them, and what North America can learn from this experience.

Evaluation of speed camera enforcement in the District of Columbia

Washington, D.C., implemented a speed camera enforcement program in 2001. Vehicle speeds were measured before and after the program. Seven sites in Washington were selected randomly from a total of 60 targeted enforcement zones. Speed data were collected 1 year before enforcement and approximately 6 months after enforcement began. Results indicated that mean traffic speeds and the proportion of drivers traveling more than 10 mph above the speed limit--fast enough to warrant a speeding ticket--were reduced at each of the Washington study sites. At eight comparison sites in nearby Baltimore, Maryland, where speed camera enforcement was not in place, no decline in traffic speeds was observed. Overall, mean speeds at Washington sites declined by a statistically significant 14% compared with Baltimore sites, and the proportion of vehicles exceeding the speed limit by more than 10 mph declined 82%. These findings indicate that speed camera enforcement reduced speeding on surface streets throughout Washington. On the basis of prior research, such speed reductions can be expected to reduce the frequency and severity of motor vehicle crashes.

Speed cameras -- public perceptions in the US

Retting, Richard A.Traffic Engineering and Control March 2003

Abstract

Public opinion regarding speed cameras is an important factor in the formulation of related traffic laws and enforcement policies. A telephone survey was conducted among 500 residents of Washington, D.C., approximately nine months after speed cameras were introduced in August 2001. Considerable awareness of speed cameras was found, and, overall, drivers favoured speed cameras rather than opposed them. Support for camera enforcement was higher among middle-aged and older drivers, among drivers who had not received a speeding ticket in the mail and did not know anyone who had, and among drivers who said speeding was a problem.

Automated enforcement myths

Fields, MicheleInsurance Institute for Highway Safety September 2000

Abstract

Traditional traffic law enforcement relies exclusively on the presence of an officer to observe violations and identify and cite offenders. Obviously, this limits the effectiveness of traffic law enforcement because police cannot be everywhere. Even when they observe violations, it is not always possible to safely stop the violator because to make the stop, the officer may have to speed or run a red light. Red light cameras and other photo-enforcement systems are designed to identify traffic law violators without depending on the presence of police officers. Red light camera systems are connected to traffic signals and to sensors buried in the pavement at the crosswalk or stop line. The system continuously monitors the traffic signal and triggers the camera to photograph the tags of vehicles entering the intersection after the light has turned red. In most cases, a second photograph is taken to show the offending vehicle in the intersection. The camera records the date, time, and speed of the vehicle; a clear image of the vehicle is produced under a wide range of light and weather conditions. Images are carefully reviewed, and citations are mailed to the registered owners of the vehicles for which there is unambiguous evidence of a violation.

Automated enforcement of traffic laws

Retting, Richard A.TR News March-April 1999

Abstract

Automated enforcement of traffic laws is increasingly being used or considered by many communities as a sensible countermeasure for deliberate traffic violations such as red light running and speeding. Automated enforcement devices can supplement limited law enforcement resources by detecting and providing evidence of such violations, thereby helping to reduce the frequency of associated motor vehicle crashes. This article reviews the use of cameras that document drivers who run red lights, violate speed limits, fail to comply with signals at railroad crossings, and operate their vehicles aggressively. Also discussed are the importance of public opinions and legal issues.

Public opinion regarding photo radar

Photo radar is an automated speed surveillance system that photographs speeding vehicles, drivers, and license plate numbers so that citations can be sent by mail. A telephone survey was conducted among residents of two communities (Paradise Valley, Arizona, and Pasadena, California) where photo radar is being used; residents of nearby communities were also surveyed. Considerable awareness of the use of photo radar was found, especially in Paradise Valley, where 72% of the people surveyed mentioned it spontaneously. In all areas, 58% either approved or strongly approved its use; residents of Paradise Valley and Pasadena were more likely to approve than residents of nearby communities. Two-thirds of those who approved of photo radar thought its use should be increased. Almost half of the respondents who knew that photo radar was being used said that they were driving more slowly as a result.

Relationship of traffic fatality rates to maximum state speed limits

Objectives: The objective of this study was to examine the safety effects of increases in U.S. state maximum speed limits during the period 1993-2013.

Methods: Poisson regression was used to model state-by-state annual traffic fatality rates per mile of travel as a function of time, the unemployment rate, the percentage of the driving age population that was younger than 25, per capita alcohol consumption, and the maximum posted speed limit on any road in the state. Separate analyses were conducted for all roads, interstates and freeways, and all other roads.

Results: A 5 mph increase in the maximum state speed limit was associated with an 8% increase in fatality rates on interstates and freeways and a 4% increase on other roads. In total, there were an estimated 33,000 more traffic fatalities during the years 1995-2013 than would have been expected if maximum speed limits had not increased. In 2013 alone, there were approximately 1,900 additional deaths — 500 on interstates/freeways and 1,400 on other roads.

Conclusions: There is a definite trend of increased fatality risk when speed limits are raised. As roadway sections with higher speed limits have become more ubiquitous, the increase in fatality risk has extended beyond these roadways. The increase in risk has been so great that it has now largely offset the beneficial effects of some other traffic safety strategies. State policymakers should keep this trade-off in mind when considering proposals to raise speed limits.

Raising the speed limit from 75 to 80 mph on Utah rural interstates: effects on vehicle speeds and speed variance

Hu, WenJournal of Safety Research 2017

Abstract

Objective: Effective May 2008, Utah allowed the speed limit for all vehicles to exceed 75 mph on rural interstate highways, following an engineering and safety assessment. Speed limits were increased from 75 to 80 mph on several roadway segments in January 2009, November 2010, and October 2013. A prior study found that passenger vehicle speeds measured 16 months after the 2009 speed limit increase were lower than before the increase, but higher than would have been expected without the increase. The current study examined the effects of the speed limit increases in 2010 and 2013 on vehicle speeds and speed variance.

Methods: Vehicle speeds were measured in May 2010 and May 2014 at sites within and near the new 80 mph speed zones and at more distant control sites where speed limits remained 75 mph. Log-linear regression models for passenger vehicles and for large trucks estimated percentage change in mean vehicle speeds associated with the speed limit increase. Logistic regression models estimated the effects of the speed limit increase on the probability of passenger vehicles exceeding 80, 85, or 90 mph and on the probability of large trucks exceeding 80 mph. The effect of the speed limit increase on speed variance also was estimated by a log-linear regression model.

Results: At sites within the 80 mph speed zones and at a nearby spillover location where the speed limit remained 75 mph, mean passenger vehicle speeds were 3.1 mph (4.1%) and 2.6 mph (3.5%) higher in 2014, respectively, than would have been expected without the speed limit increase. The probability that passenger vehicles exceeded 80 mph in 2014 was 122.3% higher than expected without the speed limit increase at sites within the 80 mph speed zones, and 88.5% higher than expected at the nearby spillover site. All increases were significant. Although not statistically significant, the probabilities that passenger vehicle speeds exceeded 85 and 90 mph were higher than expected without the speed limit increase within the 80 mph speed zones, and lower than expected at the nearby spillover site. The mean speed of large trucks and the probability that large trucks exceeded 80 mph were higher than expected within and near the 80 mph speed zones. Only the increases in mean speed were significant. The speed limit increase was associated with non-significant increases in speed variance within and near the 80 mph speed zones.

Conclusions: The current study adds to the wealth of evidence that increasing the speed limit leads to higher travel speeds and an increased probability that drivers exceed the new speed limit. Moreover, the results contradicted the claim that increasing the speed limit reduces speed variance as speed variance increased, though not significantly, after the speed limit increase. Although the estimated increases in mean vehicle speeds may appear modest, prior research suggests such increases would be associated with substantial increases in crashes, especially fatal or injury crashes. For example, an increase in mean speed from 75 to 78 mph on a freeway is associated with a 17% increase in fatal crashes, based on a model that quantified the relationship between changes in mean speed and changes in the number of crashes. This evidence should be considered by lawmakers and transportation agencies considering increasing speed limits on interstates and freeways.

Effects of vehicle power on passenger vehicle speeds

McCartt, Anne T.; Hu, WenTraffic Injury Prevention (TIP) May 2016

Abstract

Objectives: During the past two decades, there have been large increases in mean horsepower and the mean horsepower-to-vehicle-weight ratio for all types of new passenger vehicles. This study examined the relationship between travel speeds and vehicle power, defined as horsepower per 100 pounds of vehicle weight.

Methods: Speed cameras measured travel speeds and photographed license plates and the faces of drivers of passenger vehicles traveling on roadways in northern Virginia during daytime off-peak hours in spring 2013. The driver licensing agencies in the District of Columbia, Maryland, and Virginia provided vehicle information numbers (VINs) by matching license plate numbers with vehicle registration records and provided the age, gender, and zip code of the registered owner(s). VINs were decoded to obtain the curb weight and horsepower of vehicles. The study focused on 26,659 observed vehicles for which information on horsepower was available and the observed age and gender of drivers matched vehicle registration records. Log-linear regression estimated the effects of vehicle power on mean travel speeds, and logistic regression estimated the effects of vehicle power on the likelihood of a vehicle traveling over the speed limit and more than 10 mph over the limit.

Results: After controlling for driver characteristics, speed limit, vehicle type, and traffic volume, a 1-unit increase in vehicle power was associated with a 0.7% increase in mean speed, a 2.7% increase in the likelihood of a vehicle exceeding the speed limit by any amount, and an 11.6% increase in the likelihood of a vehicle exceeding the limit by 10 mph. All of these increases were highly significant. To illustrate the findings, a 3-unit increase in vehicle power, which is equivalent to the difference between the 10th and 90th percentile vehicle power for the study vehicles, is associated with a 38% increase in the likelihood that a vehicle exceeds the speed limit by more than 10 mph.

Conclusions: Speeding persists as a major factor in crashes in the United States. There are indications that travel speeds have increased in recent years. The current findings suggest that the trend toward substantially more powerful vehicles is contributing to higher speeds. Given the strong association between travel speed and crash risk and crash severity, this is cause for concern.

Raising the speed limit from 75 to 80 mph on a Utah rural interstate: effects on vehicle speeds

Objective: In January 2009, the speed limit for all vehicles was raised from 75 to 80 mph on two sections of rural interstate highway I-15 in Utah. The current study evaluated the effects of the speed limit increase on vehicle speeds.

Methods: Vehicle speeds were measured at sites within and near the 80 mph speed zones and at more distant control sites in May 2008, May 2009, and May 2010. Log-linear regression models for passenger vehicles and for large trucks estimated percentage changes in vehicle speeds associated with the speed limit increase. Logistic regression models estimated the effects of the speed limit increase on the probability of passenger vehicles exceeding 80 mph. Models of the probability of large trucks exceeding 80 mph were not estimated due to very small proportions of trucks exceeding 80 mph in 2009 and 2010.

Results: Relative to baseline speeds measured before the speed limit change, mean passenger vehicle and large truck speeds and the proportion of vehicles exceeding 80 mph declined substantially for all site groups four months after the speed limit increase took effect, with larger declines at sites with the 80 mph limit. One year later, speeds had increased somewhat at sites within or near the 80 mph speed zones, but not at control sites, and all speeds still were lower than at baseline. Based on patterns in speeds at the control sites, mean passenger vehicle speeds and the probability of passenger vehicles exceeding 80 mph generally were lower than expected in 2009 and higher than expected in 2010 at sites within the 80 mph speed zones and at nearby sites, although not all the differences were statistically significant. Notably, the odds of passenger vehicles exceeding 80 mph at sites within the 80 mph speed limit zones were estimated to be 31 percent higher in 2010 than would have been expected without the speed limit increase, a marginally significant effect. At sites near the 80 mph speed zones, there was a non-significant increase of 10.3 percent in the odds of passenger vehicles exceeding 80 mph in 2010 compared with the odds that would have been expected without the speed limit increase. Large truck speeds at sites within the 80 mph speed zones were significantly lower by an estimated 7.3 percent in 2009 and an estimated 3.7 percent in 2010, on average, than would have been expected without the speed limit increase. At sites near the 80 mph speed zones, large truck speeds were higher by an estimated 0.6 percent in 2009 and 1.6 percent in 2010, relative to expected without the speed limit increase. Only the increase in 2010 was significant.

Conclusions: Contrary to prior studies on the effects of speed limit increases in other states, observed travel speeds on affected roads in Utah decreased relative to the speeds observed before the speed limit increase in both the first and second years of the speed limit increase. The widespread reduction in speeds may reflect the effect of the unusually deep and persistent recession in the United States, along with volatile gas prices. Although speeds did not recover to their baseline levels, passenger vehicle speeds within or near the 80 mph speed zones were increasing faster than at the control sites from the first to the second year of the speed limit change, Thus, 16 months after the change, there was evidence that sections with the higher limit were encouraging faster travel than would have been expected, had the speed limit not been raised. There was no evidence that increasing the speed limit was associated with increased large truck speeds, which could reflect a greater sensitivity of these vehicles to the depths of the U.S. recession and complex and unknown ways these vehicles may have been affected.

Traffic speeds on interstates and freeways 10 years after repeal of national maximum speed limit

Objectives: Essentially all published analyses of operational and safety outcomes related to enactment or repeal of the national maximum speed limit (NMSL) were based on data limited to the initial 1-3 years. The purpose of the present study was to collect and analyze longer term speed data.

Method: In 2006, traffic speeds were surveyed at 26 locations on urban and rural expressways in five states (California, Montana, Nevada, New Mexico, and Texas) where speeds had been measured in 1996, immediately after repeal of NMSL. Most speed limits were unchanged during the approximately 10-year period. However, Montana introduced a numeric 75 mph limit for passenger vehicles in place of a "reasonable and prudent" limit. In Texas, urban freeway speed limits for passenger vehicles were reduced 5 mph, and truck limits were increased 5 mph.

Results: On rural interstates without speed limit changes, travel speeds increased for both passenger vehicles and large trucks; the proportion of passenger vehicles exceeding 80 mph tripled. On rural interstates in Montana where speed limits were lowered for passenger vehicles, travel speeds decreased, even for large trucks whose speed limits had not changed. On urban freeways where speed limits did not change, travel speeds declined somewhat for both passenger vehicles and large trucks; during the study period there also were large increases in traffic volume and development of surrounding areas. On urban freeways in Texas where speed limits declined for passenger vehicles, travel speeds generally decreased, even for large trucks whose speed limits actually had increased.

Conclusions: The data suggest that where traffic volumes allow, travel speeds still are increasing 10 years after repeal of NMSL, and many drivers are speeding. The study also shows that speed increases can be curbed and even reversed when speed limits are lowered. Increased enforcement is needed to curb speed increases and the increased risk of serious crashes.

Problem: In 2006 Texas raised the daytime speed limit for passenger vehicles on segments of I-10 and I-20 from 75 to 80 mph.

Methods: Traffic speeds were measured before and 3, 12, and 16 months after the limit was changed.

Results: During the 16-month period following the speed limit increase, mean speeds of passenger vehicles on I-20 increased by 9 mph relative to the comparison road, where no speed limit change occurred and traffic speeds declined. On I-10 mean speeds increased by 4 mph relative to the comparison road. Limiting the analysis to the month before the speed limit change and 1 year later, the proportion of drivers exceeding 80 mph was 18 times higher on I-20 and 2 times higher on I-10.

Discussion: The smaller speed increases on I-10 may be related to its proximity to the U.S. border with Mexico. Highly visible border patrol activity coincided with posting of the higher speed limit. Long-term monitoring in other states suggests that traffic speeds in Texas are likely to continue to increase.

Impact on industry: The present study adds to the wealth of evidence that increased speed limits lead to increased travel speeds. The primary countermeasures to reduce the risk of speed-related crashes include highly visible police traffic enforcement and the use of speed cameras accompanied by publicity.

Characteristics of speeders

Objective: To determine the characteristics of speeders, defined as drivers of vehicles traveling at least 15 mph above the posted speed limit and relatively faster than surrounding vehicles.

Methods: Vehicle speeds were recorded on 13 roads in Virginia with speed limits ranging from 40 to 55 mph. Speeders were compared with slower drivers, defined as drivers of adjacent vehicles traveling no more than 5 mph above the speed limit. License plates were used to identify vehicle owners; owners were inferred to have been driving if observed gender and estimated age matched those of the registered owner. For these drivers, information on exact driver age and gender, vehicle make and model, and driving record was obtained from the Virginia Department of Motor Vehicles.

Results: Five percent of the vehicles observed were traveling at least 15 mph above the limit, and 3% qualified as speeders, as defined in this study. Speeders were younger than drivers in the comparison group, drove newer vehicles, and had more speeding violations and other moving violations on their records. They also had 60% more crashes.

Discussion: Speeders are a high-risk group. Their speeding behavior is not likely to be controlled without vigorous, consistent enforcement, including the use of automated technology.

Changes in motor vehicle occupant fatalities after repeal of the national maximum speed limit

Trends in motor vehicle occupant deaths over 8 years were studied for 24 states that raised interstate speed limits and seven states that did not following the 1995 repeal of the US National Maximum Speed Limit. Fatalities on interstates increased 15% in the 24 states that raised speed limits. After accounting for changes in vehicle miles of travel, fatality rates were 17% higher following the speed limit increases. Similar increases were reported following the 1987 speed limit increases on rural interstates. Deaths on roads other than interstates were essentially unchanged.

Use of pavement markings to reduce excessive traffic speeds on hazardous curves

Retting, Richard A.; Farmer, Charles M.ITE Journal September 1998

Abstract

This feature reports on a research experiment to examine the effectiveness of special pavement markings intended to reduce excessive traffic speeds at rural and suburban two-lane roadway locations with sharp horizontal curvatures. The experiment was conducted at a single location on a suburban two-lane secondary road in northern Virginia that includes a sharp left curve--approximately 90 deg (1.6 rad)--preceded by a long tangent section. An advisory speed of 15 mph (24 kph) is posted approximately 500 ft (152 m) before the curve using a standard reverse-turn sign and advisory speed plate. The experimental pavement marking was intended to reinforce this existing advisory and consisted of the word "SLOW" in 8-ft (2.4-m) high white letters, a white 8-ft (2.4-m) high left curve arrow, and an 18-in (46-cm) wide white line perpendicular to the road at both the beginning and end of the text/symbol message. A separate left curve--approximately 45 deg (0.8 rad)--in the opposite direction of the same highway, approximately one-quarter mi (0.4 km) away was chosen as a control site. At the experimental site, traffic speeds were measured on the tangent section 90 ft (27 m) prior to the point of curvature before and after installation of the pavement marking. At the upstream site, traffic speeds were measured approximately 650 ft (198 m) prior to the point of curvature. At the experimental site, the mean traffic speed dropped from 34.3 mph (55.2 kph) during the baseline to 33.2 mph (53.4 kph) after marking. In contrast, speeds increased at both the upstream and control sites. The percentage of vehicles exceeding 40 mph (64 kph) on the experimental curve dropped by more than half, from 9.1% to 3.5%.

The number of motor vehicle occupant fatalities in 1996 was compared with those in each year during 1990-95. Deaths in 12 states that raised speed limits to 70 mph (113 kph) on at least some roads in December 1995 or early 1996 were compared with deaths in a group of states that did not. On rural and urban interstates and freeways, where most of the speed limit changes occurred, speed limit increases were associated with a 12% increase in occupant fatalities. On other roads, where speed limits were raised less frequently, occupant fatalities increased by only 3% (nonsignificant). On all roads combined, occupant fatalities increased by 6%. It is estimated that during the last 9 months of 1996 there were 500 additional deaths in the 12 states that raised speed limits.

Traffic speeds following repeal of the national maximum speed limit

Retting, Richard A.; Greene, Michael A.ITE Journal May 1997

Abstract

New traffic speed limits in interstate highways, following the repeal of the national maximum speed limit, increased the proportion of overspeeding vehicles. This has become one of the most prevalent factors contributing to motor vehicle crashes. It is proposed that states that increase speed limits should undertake countermeasure to limit potential harm which include ample speed enforcement, safety belt use, and upgrading of the design standards of highway facilities to better accommodate higher traffic speeds.

Faster but deadlier

Williams, Allan F.Recovery Summer 1996

Abstract

Studies have shown that most drivers exceed posted speed limits. So why not simply raise speed limits to match the flow of traffic? In the United States, two periods of widespread change in the posted limits on high-speed routes have allowed researchers to assess the effects of different limits on roadway losses. The conclusion is simple: increases in posted speed limits are soon followed by increases in roadway crashes and fatalities.

Effect of radar drone operation on speeds at high crash risk locations

At highway construction and maintenance work zones and other locations where roadway alinement, road surface, and traffic flow conditions have contributed to high crash rates, crash risk may be reduced by lower and more uniform speeds. The use of unattended (drone) radar has been found to reduce the mean speed of vehicles and the number traveling at very high speeds. Drivers using radar detectors to warn of police speed enforcement activities respond to the warning and slow down, as do drivers of nearby vehicles. Speeds were measured with and without radar drones in operation at 12 construction and maintenance work zones and high crash locations in Missouri. It was found that mean speeds were moderately lower when radar was operating, and this effect was slightly greater for tractor-trailers than for passenger vehicles, although not significantly so at most locations. However, moderate reductions in mean speed were associated with more meaningful reductions in the number of vehicles exceeding the speed limit by more than 10 mph (17 kph), especially among tractor-trailers.

Response to "Intervention analysis for the impacts of the 65 mph speed limit on rural interstate highway fatalities" by Chang, Chen and Carter

In "Intervention Analysis for the Impacts of the 65 mph Speed Limit on Rural Interstate Highway Fatalities," Chang, Chen, and Carter claim that the 65 mph speed limit initially increased highway fatalities but that this effect decayed after one year. These conclusions are contrary to Chang et al.'s own results as well as the vast majority of the published literature on the effects of raising and lowering the national maximum speed limit. The approaches and assumptions in this work are flawed and the conclusions should be dismissed.

The effects of mobile roadside speedometers on traffic speeds

A study was conducted to evaluate mobile roadside speedometers as a means of controlling urban traffic speeds under varied schedules of deployment and speed limit law enforcement. Speeds of cars passing the roadside speedometer were measured using nondetectable radar. The data indicate that, generally, the speedometer's presence reduced average traffic speeds by about 10% alongside the speedometer and about 7% at short distances downstream. The proportion of drivers exceeding the speed limit by at least 10 mph fell dramatically from 15%-20% to only 2% at one site on days the speedometer was deployed, and the device was particularly effective when deployed in school zones. However, the effect of the speedometer was limited to the times when it was actually deployed. Associated police enforcement is a key factor, as the effect of the speedometer decayed over time but could be long lasting with a minimal amount of enforcement activity in the area of the speedometer.

The effect of laser speed-measuring devices on speed limit law enforcement in Charleston, South Carolina

Teed, Nancy J.; Lund, Adrian K.Accident Analysis and Prevention 1993

Abstract

Drivers exceeding the speed limit by more than 10 mph were identified and ticketed by the Charleston County, South Carolina police using two types of speed measuring devices: conventional police radar and a new laser device. The two types of enforcement were used alternately on the same roads at similar times. Under the laser enforcement condition, the police issued 534 tickets (54%) compared with 457 (46%) under radar enforcement. The increase in tickets issued under laser enforcement was observed at three of the four study sites, and the number of tickets per traffic volume increased at two of the three sites for which traffic volumes were available. Speeders ticketed under the laser enforcement condition were four times as likely to have radar detectors as those ticketed under the radar enforcement condition. Most of the additional speeders caught by the laser were using radar detectors. These data indicate that some speeders with radar detectors are avoiding detection when the police enforce the limit with radar.

Response to "Safety impacts of the 65 mph speed limit on interstate highways" by Chang, Carter, and Chen

The paper, "Safety Impacts of the 65 mph Speed Limit on Interstate Highways," published by the AAA Foundation for Traffic Safety claims that the 65 mph speed limit initially increased highway fatalities but that this effect decayed after one year and that the increase in fatalities was far less than found in other studies. These conclusions are contrary to Change et al.'s own results as well as those of numerous other studies. The conclusions reported by Chang et al. were based on a highly selective review of the literature, incorrect counts of fatalities, and inadequate study design and analyses. The approaches and assumptions in this work are flawed and the conclusions should be dismissed.

Speeds associated with 55-mph and 65-mph speed limits in Northeastern states

Freedman, Mark; Williams, Allan F.ITE Journal February 1992

Abstract

Surveys have shown that raising the speed limit from 55 mph to 65 mph produces higher average speeds and a larger percentage of vehicles traveling at very high speeds. As a consequence of the 65 mph speed limit, fatalities on rural interstates have increased 20-30%. This paper describes a study in which information on speeds in various states was obtained by measuring vehicle speeds on rural interstate highways in 6 states that have retained 55 mph limits and 5 adjacent states with 65 mph limits. Details of the data collection and analysis are summarized. A summary of free-flowing vehicle speeds for passenger cars and tractor-semitrailers and the percentage of vehicles exceeding 65, 70, and 75 mph on the studied rural interstate highways in each state is tabulated. The results showed clearly, that the proportions of vehicles traveling at high speeds are substantially lower in states with 55 mph speed limits than in states with 65 mph limits. The details of the study results are discussed.

Police enforcement resources in relation to need: changes during 1978-89

A survey of state highway and city police agencies was conducted to determine whether the relationship between police staffing and measures of need for police traffic services has changed over time. Data on staffing levels, the number of vehicle miles traveled, and number of licensed drivers were obtained for the years 1978, 1981, and 1984 through 1989 for 33 of the 49 surveyed states. Staffing level for five cities, and daily vehicle miles of travel for the associated urbanized areas, were obtained for 1981 and 1984-89. For the state highway police agencies, the number of vehicle miles traveled and number of licensed drivers rose faster than the availability of officers whose routine duties included traffic law enforcement; in a few states the number of state traffic officers has actually declined. Three of the cities have shown substantial declines in the ratio of traffic officers to area travel. In most states and cities, the data indicate that the ability of police agencies to respond to the rising need for traffic enforcement has diminished over time. This suggests that more efficient allocation of police resources, such as the use of automated enforcement techniques for speed and signal light violations, is needed in many places.

Changes in speed and speed adaptation following increase in national maximum speed limit

Casey, Steven M.; Lund, Adrian K.Journal of Safety Research 1992

Abstract

This study replicates a 1985 study showing that drivers adapted to freeway speeds travel faster on roads connecting with the freeways than do other drivers on the same roads. In addition, the study indicates that travel speeds on these freeways were faster in 1988 even though they had retained the 55 mph (88 ) limit and were distant from highways authorized for the 65 mph (105 ) limits. These findings suggest that allowing higher speeds on some highways not only causes higher speeds on local, connecting roads through speed adaptation, but also may cause higher speeds on other, unconnected and distant roads through some indirect process of speed generalization.

The fatality consequences of the 65 mph speed limits, 1989

This study examined whether the number of fatalities on rural interstates in 1989 was higher than would be expected based on experience during 1982–1986 and experience on all other roads. Among the 40 states that increased the speed limit to 65 mph on rural interstates, the number of fatalities was 29% higher than expected. Among the eight states retaining a 55 mph maximum speed limit on rural interstates, the observed number of fatalities was 12% lower than expected, although this reduction was not statistically significant. After adjusting the fatality risk on rural interstates for differences in vehicle miles traveled on those roads and for higher passenger vehicle occupancy rates attributable to possible increases in vacation travel, the increased fatality risk was 19%. These data suggest that the majority of the estimated increase in fatalities on rural interstates in 1989 (almost 400 of the approximately 600 extra deaths) can be attributed to the higher speeds resulting from the higher speed limits. Changes in mileage account for the remaining 200 extra deaths.

A critique of Tignor and Warren's "Driver speed behavior on US streets and highways"

The paper "Driver Speed Behavior on U.S. Streets and Highways," by the Federal Highway Administration's (FHWA's) Samuel Tignor and Davey Warren, erroneously concludes that raising speed limits by up to 15 mph has little or no effect on travel speeds, and that speed limits should be set in the 70-90th percentile range (Tignor and Warren, 1990). These conclusions are contrary to the findings of numerous other studies that show that speeds increase when speed limits are raised. Moreover, the studies used by Tignor and Warren to support their claims have not been released by the FHW A, preventing proper peer review of the research. Evidence available from unpublished copies of the research reports shows that the conclusions arrived at by Tignor and Warren are unwarranted. Tignor and Warren's paper could promote public policy and traffic engineering actions that are likely to result in more crashes and crash deaths. The paper and its suggestions regarding raising speed limits do a dangerous disservice to traffic engineering and highway safety.

The paper entitled, "Did the 65 mph Speed Limit Save 3,113 Lives," by Charles Lave claims "...the states which adopted the 65 mph speed limit (on rural interstates) saved 3,113 lives in 1987 and 1988, compared to states that did not change the speed limit." If this claim were true, the higher speed limit would have saved almost as many lives as were lost on rural interstates in the United States in 1986, the year before any speed limit increases. Lave attempts to justify this claim with the supposition that higher speed limits on rural interstates led to a reallocation of police enforcement resources to other roads and to other problems although there is scant evidence for this supposition. Lave's catchy title and absurd claim may be good theater, but they contradict the finding of every published evaluation of the effect of raising the rural interstate speed limit to 65 mph -- increasing the speed limit has increased fatalities. Lave's analytic approach has no scientific validity and his conclusions must be dismissed as nonsense.

Different speed limits for cars and trucks: do they affect vehicle speeds?

During 1987 and 1988, 40 states opted to take advantage of the 1987 federal law allowing them to raise the speed limit on rural Interstates from 55 to 65 mph. The majority of these states raised the limit to 65 mph for all vehicles; however, 10 states chose a lower speed limit for trucks than for cars. Vehicle speeds were measured on rural Interstates in California and Illinois, which have a differential speed limit, and in Arizona and Iowa, which have a uniform speed limit. A posted differential speed limit on rural Interstates was found to reduce high truck speeds on the faster roads. Trucks are a smaller percentage of the high-speed traffic in states with differential speed limits than in states with uniform speed limits when average car speeds exceed 63.4 mph. Specifically, for each 1-mph increase in mean car speed over 63.4 mph on rural Interstates, the odds relative to cars of a truck traveling about 70 mph decreases by 20% in the states with differential speed limits compared with states having uniform speed limits. Analysis of the mean speeds revealed that trucks travel 1.4 mmph slower in states with differential speed limits than in those without. This difference increases to 3.0 mph for the fastest 5% of trucks.

The purpose of this study was to estimate the effect of 65 mph speed limits in all 40 states with the higher limits during 1989. The data source used was the Fatal Accident Reporting System of the National Highway Traffic Safety Administration. It was found that the number of fatalities on rural interstate highways increased by 32% in 1989 compared to 1982-86. In contrast, the number of fatalities on all other roads only increased by 2% during this same period. Similarly, the number of fatalities on urban interstate highways increased by 8%. The estimated risk for a rural interstate fatality increased by 29% in 1989 relative to all other roads and increased by 22% relative to only urban interstates. Fatalities for drivers and occupants of passenger vehicles were examined separately. The relative risk of fatalities on rural interstates was also significantly higher for drivers and occupants of passenger vehicles in 1989. Among the eight states that maintained a 55 mph speed limit, the number of rural interstate fatalities in 1989 was 10% lower than the average for 1982-86. Over the same period the number of fatalities on all other roads increased by 3% and on urban interstates they increased 15%. The estimated risk of a rural interstate fatality declined 22% relative to urban interstates and 12% relative to all other roads. These results indicate that the 65 mph speed limits resulted in an increased risk of a fatality on rural interstates in the 40 states with higher speed limits. For all fatalities, the increased risk associated with the higher speed limit is 22-29%.

Experience with speed limits in the USA

Setting speed limits in the United States historically has been a state responsibility. In 1974, however, a national maximum speed limit of 55 mph was established. Originally enacted as a temporary measure to save fuel, the United States Congress continued this limit because of its obvious safety benefits. Immediately after the imposition of the 55 mph limit, both highway speeds and the number of motor vehicle deaths dropped. In 1973, there were 55,511 deaths and this number declined to 46,402 in 1974. Not all of this fatality reduction can be attributed to the lower travel speeds because mileage also declined and patterns of travel changed. There is no question, however, that the lower travel speeds were major contributors to the fatality decline.

Motor vehicle crash fatalities in the second year of 65 mph speed limits

In 1987, the federal government relaxed the 55 mph (88 km/h) national maximum speed limit, permitting states to post 65 mph (104 km/h) speed limits on rural interstate highways. By the end of 1987, 38 states had done so; and two additional states followed suit in 1988. It is estimated that the higher speed limit caused motor vehicle crash fatalities to be 26% higher in 1988 than they would have been if the speed limit had remained at 55 mph. This translates into over 500 deaths attributable to the higher speed limits on rural interstate highways. Overall, it is estimated that more than 700 people, who would have lived had the limit not been raised, have died on rural interstate highways since states began to post 65 mph limits in 1987. The effect of the higher limit doubled between 1987 and 1988, and the 65 mph limit may cost far more in death and injury than predicted prior to the speed limit increase.

Effect of the 65-mph speed limit on speeds in three states

Following the April 1987 enactment of federal law permitting 65-mph speed limits on rural Interstate highways, 40 states adopted higher speed limits by the middle of 1988. Nondetectable radar was used to measure speeds in three states to evaluate the effect of the 65-mph speed limit on speeds of free-flowing vehicles on Interstate highways during daytime off-peak periods (9:00 a.m. to 4:00 p.m.). In New Mexico, rural and urban speeds were measured at 2-month intervals over a 2-year period after the speed limit was increased in April 1987. In Virginia and Maryland, rural speed data were collected immediately before and after Virginia implemented the 65-mph limit in July 1988 and data collection was repeated at 3-month intervals for 1 year. Two weeks after the 65-mph speed limit began in Virginia, mean and 85th-percentile speeds of cars were higher by almost 3 mph, whereas the speed of tractor-trailers (still limited to 55 mph) was unchanged. The proportion of cars exceeding 70 mph nearly doubled. Speeds of cars and trucks in neighboring Maryland (with 55-mph speed limit) did not increase during the same 2 weeks. A longer-term trend of increasing speed was also found in Virginia. In contrast, car speeds in Maryland showed no upward trend, but tractor-trailer speeds have increased to the same level as in Virginia. In New Mexico, average speeds of passenger cars and light trucks on rural highways increased nearly 3 mph within 9 months of the 65-mph law and have since continued to increase. The proportion exceeding 70 mph grew nearly fivefold for cars and doubled for heavy trucks. Urban highway speeds in New Mexico have shown a slight increase over 27 months, while also exhibiting pronounced seasonal variation.

The mortality consequences of raising the speed limit to 65 mph on rural interstates

As of April 1987, states were permitted to raise the speed limit on rural interstates to 65 mph without incurring federal sanctions; 38 states elected to do so in 1987. Fatality data for the months when the new limit was in effect in 1987 were compared with fatalities in the same months of 1982-86 on rural interstates and other rural roads. Fatalities on rural interstates in the states with increased speed limits in 1987 were conservatively estimated to be 15 percent higher than they would have been if the states had retained the 55 mph limit (95% CI = 6, 24). Among states that retained the 55 mph limit, fatalities on rural interstates were 6 percent lower than expected (95% CI = -23, 13).

A critique of "Highway safety and the 65 mph maximum speed limit: an empirical study"

Three field studies of driver speed adaptation

Casey, Steven M.; Lund, Adrian K.Human Factors October 1987

Abstract

Three field studies of driver speeds were conducted to test the speed adaptation phenomenon and to define the practical implications of its effect. Sites were selected in which the speeds of vehicles previously exposed to high-speed conditions could be contrasted with speeds of vehicles not previously exposed to high speeds. The following conclusions were drawn from the results of this study: Conditions specific to a traffic site, such as legal speed limits, traffic density, and cross-street activity, determine the extent of speed adaptation. Speed perpetuation does not account for observed speed differences between speed-adapted and non-speed-adapted vehicles. The effects observed in the present study were significant but lower than in previous studies, possibly because of overall lower vehicle speeds. These findings indicate that proposals to increase speeds on rural interstates are likely to result in higher speeds on other, connecting roads as well.

The 55 mph limits and front-to-rear collisions involving autos and large trucks

The effects of the establishment of 55 mph limits on front-to-rear crashes involving automobiles and trucks were examined. Since the establishment of 55 mph limits resulted in a reduction in the difference between the reported average speed of automobiles and large trucks, it provided an opportunity to examine the effects of speed differences on the frequency of crash involvement of these vehicles. Principal results of the study were as follows: In 1974, the year of the introduction of 55 mph speed limits, a substantial reduction in the number of front-to-rear crashes involving an automobile and tractor trailer on higher speed roads occurred in the states whose experience was examined. A substantial decline in the number of front-to-rear crashes involving an auto and single body truck on higher speed roads also occurred in 1974 in these states. The decline in the number of front-to-rear crashes involving an automobile and tractor trailer on higher speed roads was primarily the result of a major decline in the number of crashes in which an auto struck a tractor trailer in the rear. The number of crashes in which a tractor trailer struck an auto in the rear declined by a much smaller percentage. The decline in the number of front-to-rear crashes involving an automobile and single body truck on higher speed roads resulted from comparable decreases in the number of crashes in which an auto struck a single body truck in the rear and those in which a single body truck struck an auto in the rear. Prior to the establishment of 55 mph limits, tractor trailers struck automobiles in the rear in more than half of the front-to-rear crashes involving these vehicles on both higher and lower speed roads. Because the major decline in the number of crashes in which autos struck tractor trailers in the rear following the introduction of the new limits was not matched by as large a decline in the number of crashes in which tractor trailers struck autos in the rear, there was a significant increase in the proportion of front-to-rear crashes involving an automobile and tractor trailer in which the tractor trailer struck the automobile in the rear on higher speed roads. Prior to the establishment of 55 mph limits, single body trucks struck automobiles in the rear in more than half of the front-to-rear crashes involving these vehicles on both higher and lower speed roads. However, single body trucks struck autos in the rear in a lower proportion of their front-to-rear crashes with autos than did tractor trailers. The proportion of front-to-rear crashes involving an automobile and single body truck in which an auto was struck in the rear by a single body truck was not significantly affected by the establishment of 55 mph limits.